The electrolytic production of chlorine and caustic by the electrolysis of brine has been well known for many years. Historically, diaphragm cells using a hydraulically-permeable asbestos diaphragm, vacuum-deposited onto foraminous steel cathodes, have been widely commercialized. Such diaphragm cells, employing permeable diaphragms, produce NaCl-containing NaOH catholytes because NaCl passes through the diaphragm from the anolyte to the catholyte. Such NaCl-containing caustic generally requires a de-salting process to obtain a low-salt caustic for industrial purposes.
In recent years, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low-salt of salt-free caustic in order to improve quality and avoid the costly desalting processes. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated Na.sup.+ ions to be transported from the anolyte portion to the catholyte portions, while substantially preventing transport of Cl.sup.- ions. Such cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. The anodic reactions and cathodic reactions are the same regardless of whether a membrane cell or a diaphragm cell is employed.
Since the disclosure of fluorocarbon polymers containing sulfonic acid functional groups on pendant fluorocarbon chains was first disclosed by Connolly (U.S. Pat. No. 3,282,875), a great deal of work has been done on using these and similar materials as ion exchange membranes in chloralkali cells. it has been stated that because of excessive hydration, sulfonic acid membranes are not useful, particularly at cell conditions where the caustic strength in the operating cell exceeds 18% (Maomi Seko, Commercial Operation of the Ion Exchange Membrane Chlor-Alkali Process, The American Chemical Society Meeting, April, 1976, New York, N.Y.). Because of the problems encountered with sulfonic acid substituted membranes, considerable work has been directed at finding a suitable substitute for the sulfonic acid. Carboxylic acid functional membranes have been reported to operate at considerably higher caustic strengths than sulfonic acid membranes (M. Seko above ref., U.S. Pat. No. 4,065,366, Brit. Pat. Nos. 1,497,748; 1,497,749; 1,518,387). Membranes with at least part of the sulfonic acid groups converted to sulfonamide have also been reported to operate at higher caustic strengths than membranes with only sulfonic acid functional groups (U.S. Pat. Nos. 3,784,399; 3,969,285). The incentive for striving for high caustic strength in the cell lies in the fact that most commercial caustic is sold as a 50% solution. Thus, higher strengths achieved in the cell results in less water that must be evaporated to reach the commercial 50% level. This results in savings of "evaporation energy", the energy required to evaporate the solution.
In addition to the caustic strength being important, two other criteria of the operating cell must also be considered for a complete energy view of the overall process. One is current efficiency, which is the ability of the membrane to prevent migration of the caustic produced at the cathode into the anode compartment and the second is the voltage at which the cell operates, which is partly determined by the electrical resistance of the membrane. Power efficiency is often used as one term that considers both the current efficiency and cell voltage. It is defined as the product of the theoretical voltage divided by the actual voltage multiplied by the actual caustic produced divided by the theoretical caustic that could have been produced at a given current. Thus, it is apparent that power efficiency is reduced by higher cell voltage or lower current efficiency. The membrane has a direct effect on both. The most common method of comparing cells is to express the operation as kilowatt hours (KWH) of power consumed per metric ton (mt) of product produced. This expression also considers both voltage, higher voltage increasing the quantity KWH, and current efficiency, lower efficiency decreasing the quantity of product produced (mt). Thus, the lower the value KWH/mt, the better the performance of the cell.
In general, the changes that have been made in membranes to increase the caustic strength in the cell have resulted in at least partially offsetting increases in the quantity KWH/mt. It has been reported that even though a carboxylic acid membrane was capable of producing greater than 30% caustic at above 90% current efficiency, the most economical operation was at 21-25% caustic because of lower cell voltage (M. Seko, "The Asahi Chemical Membrane Chlor-Alkali Process", The Chlorine Institute, Inc. 20th Chlorine Plant Managers Seminar, New Orleans, February, 1977). In addition to problems of increasing cell voltage caused by membranes capable of higher caustic strength operation, these types of membranes, when compared to sulfonic acids, do not last as long in service. This is at least in part caused by greater sensitivity than the sulfonic acids to impurities found in brine feed. It has been reported that the useful operating life of sulfonamide membranes is only about one year (D. R. Pulver, presented at the Chlorine Institute's 21st Plant Managers Seminar, Houston, Tex., Feb., 1978). Sulfonic acid membranes have operated up to three years in chlor-alkali cells. A great deal of expense is incurred by shortened membrane life because of having to replace the expensive membrane materials. Also the loss of production and labor in having to remove cells from service, disassemble, assemble and put them back in service in costly.
The polymers used in the prior art as membranes are generally copolymers formed by copolymerizing a monomer chosen from the group of fluorinated vinyl compounds composed of vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifuloroethylene, chlorotrifluoroethylene, perfluoroalkyl vinyl ether and tetrafluoroethylene with an ion exchange functional (or group easily converted to ion exchange functional) vinyl ether monomer. The functional monomers for the sulfonic acids of the prior art are represented by U.S. Pat. No. 3,282,875) ##STR1## The sulfonamide of the prior art are represented by the general structure ##STR2## where the FSO.sub.2 -group is converted to RNHSO.sub.2 -by reaction with an amine compound and n=0-2 (Brit. No. 1,406,673 and U.S. Pat. No. 3,784,399). The carboxylic acid monomers are represented by similar structures where the sulfonyl group has been replaced with either a carboxylic acid or a group such as ##STR3## or --C.tbd.N that is easily converted to a carboxylic acid (U.S. Pat. No. 4,065,366, Brit. Nos. 1,497,748; 1,497,749; 1,518,387). In one case (U.S. Pat. No. 4,126,588), the membrane is composed of a terpolymer made by selecting one monomer from the group of perfluorovinyl compounds listed above and the other two from different carboxylic acid functional monomers. One is chosen from a group represented by CF.sub.2 .dbd.CFOCF.sub.2 (CFXOCF.sub.2) (CFX').sub.B -(CF.sub.2 OCFX").sub.Y -A where A represents a carboxylic acid or derivative and the other from a group represented by CF.sub.2 .dbd.CF(O)-(CFY).sub.y -A' where A' is defined as A above. Two different functional monomers were used in the above case to achieve desirable physical properties of the polymers.
In addition to work described above where changes in functional groups have been used as a means of achieving higher caustic strength in operating cells, methods of operating the cells themselves that lead to increased caustic strength have been described. Series catholyte flow (U.S. Pat. No. 1,284,618) and countercurrent series anolyte and catholyte flow (U.S. Pat. No. 4,197,179) lead to increased caustic strength without sacrificing either current efficiency or cell voltage. These techniques are also useful because caustic strengths approaching those obtained with carboxylic acid and sulfonamide membranes can be attained using sulfonic acid membranes with their inherently longer service life.
U.S. Pat. Nos. 4,025,405 and 4,192,725 show electrolytic cells having a stable, hydrated, selectively permeable, electrically conductive membrane. The membrane is a film of fluorinated copolymer having pendant sulfonic acid groups containing recurring structural units of the formula: ##STR4## in which R' is F or perfluoroalkyl of 1 to 10 carbon atoms; Y is F or CF.sub.3 ; m is 1, 2, or 3; n is 0 or 1; X is F, Cl, H, CF.sub.3 ; X' and X are CF.sub.3 -(CF.sub.2).sub.z wherein Z is 0-5; the units of formula (1) being present in an amount of from 3-20 mole percent.